Analysis of current conduction mechanisms in atomic-layer-deposited Al 2O3 gate on 4H silicon carbide

Article abstract of DOI:10.1063/1.2730731

Current conduction mechanisms of an atomic-layer-deposited A1 ₂O₃ gate on n-type 4H SiC have been systematically investigated, analyzed, and reported in this letter. It has been revealed that space charge limited, Poole-Frenkel (PF) emission, combination of PF emission and Fowler-Nordheim tunneling are the dominate current conduction mechanisms in the dielectric. Besides, Schottky emission has also been proposed as a possible leakage path at temperature beyond the investigated range. A relationship among the conduction mechanism, temperature, and applied electric field has been presented.

Full text of DOI:10.1063/1.2730731

Analysis of current conduction mechanisms in atomic-layer-deposited Al 2 O 3 gate on
4 H silicon carbide
Kuan Yew Cheong, Jeong Hyun Moon, Hyeong Joon Kim, Wook Bahng, and Nam-Kyun Kim
Citation: Applied Physics Letters 90, 162113 (2007); doi: 10.1063/1.2730731
View online: http://dx.doi.org/10.1063/1.2730731
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APPLIED PHYSICS LETTERS 90, 162113 ͑2007͒
Analysis of current conduction mechanisms in atomic-layer-deposited
Al2O3 gate on 4H silicon carbide
Kuan Yew Cheonga͒
School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia,
14300 Nibong Tebal, Seberang Perai Selatan, Penang, Malaysia
Jeong Hyun Moon and Hyeong Joon Kim
School of Materials Science and Engineering, College of Engineering, Seoul National University,
Seoul 151-744, Korea
Wook Bahng and Nam-Kyun Kim
Semiconductor Power Research Group, Korea Electrotechnology Research Institute, P.O. Box 20,
Changwon, Gyungnam 641-120, Korea
͑Received 25 January 2007; accepted 19 March 2007; published online 20 April 2007͒
Current conduction mechanisms of an atomic-layer-deposited Al2O3 gate on n-type 4H SiC have
been systematically investigated, analyzed, and reported in this letter. It has been revealed that space
charge limited, Poole-Frenkel ͑PF͒ emission, combination of PF emission and Fowler-Nordheim
tunneling are the dominate current conduction mechanisms in the dielectric. Besides, Schottky
emission has also been proposed as a possible leakage path at temperature beyond the investigated
range. A relationship among the conduction mechanism, temperature, and applied electric field has
been presented. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2730731͔
Usage of high-dielectric constant ͑␬͒ gate oxide on sili-
con carbide ͑SiC͒ based metal-oxide-semiconductor ͑MOS͒
devices has gained much attention recently due to its en-
hanced electrical strength and stability.1–8 Of many high-␬
gate oxides, Al2O3 is one of the most extensively investi-
gated insulators owing to its compatible intrinsic properties
with SiC substrate.4–8 It has been reported that atomic-layer-
deposited Al2O3 ͑electric field Eϳ14 MV/cm ͓equivalent
oxide thickness ͑EOT͒=6 nm͔ or Eϳ6.3 MV/cm ͑physical
oxide thickness tox=13 nm͒ has a higher hard breakdown
field ͑EHBD͒ and lower leakage current than its nitrided SiO2
͓Eϳ10 MV/cm ͑tox=15 nm͔͒ counterpart.8,9 The leakage
current and dielectric breakdown in Al2O3 gate are strongly
affected by the way current conducts through the oxide and
this is directly influenced by SiC substrate surface treatment
prior to deposition, gate oxide deposition technique, and
postdeposition heat treatment method.4–8 Huang and Hwu5
have demonstrated that Schottky emission ͑SE͒ is the domi-
nating current conduction mechanism ͑2.5 MV/cmϽE
Ͻ11.9 MV/cm, 423 Kഛtemperature ͑T͒ഛ523 K͒ in a
nitric-acid oxidized Al2O3 gate ͑EOT=2.6 nm͒. In contrast,
function of T and E, of an atomic-layer-deposited Al2O3 gate
on n-type 4H SiC will be analyzed and reported.
1 cm2 diced n type, 8° off ͑0001͒ oriented, and
10-␮m-thick epilayer 4H-SiC wafers with ͑1–4͒
ϫ1016 cm−3 dopant of nitrogen were used to fabricate the
MOS-capacitor test structures. After undergoing a standard
wafer cleaning process with a final 2 min HF dip, the diced
wafers were inserted into a custom-made atomic layer depo-
sition chamber. Trimethyl aluminum and water were, respec-
tively, employed as precursor and oxidant for the deposition
of 13-nm-thick Al2O3. The comprehensive deposition, test
structure fabrication procedures, and electrical properties of
the deposited oxide have been reported elsewhere.8,10,11
A
semiconductor parameter analyzer ͑HP-4156͒ was used to
measure leakage current of the MOS capacitor with an area
of 1.30ϫ10−3 cm2 at various temperatures ͑293–413 K͒.
The gate current ͑IG͒ as a function of forward gate-voltage
sweep, VG, ͑ramping rate=0.3 V/s͒ was recorded.
In order to analyze the current conduction mechanisms
of the Al2O3 gate, many conduction mechanisms have been
evaluated. Experimental results show that current conducts
through the Al2O3 gate, following Lampert’s theory of space-
charge-limited ͑SCL͒ mechanism at low VG region. Three
distinct and visible regions with different slopes ͑S͒ in J-VG
plot have been observed in all the investigated temperatures,
except for sample tested at 293 and 413 K ͑inset of Fig. 1͒.
These three regions are confined in a triangle, governed by
Ohm’s law ͓JOh2mᐉ+=1 qn0␮͑VG/tox͔͒, trap-filled limited ͑TFL͒
Fowler-Nordheim
͑FN͒
tunneling
͑3.6 MV/cmϽE
Ͻ16 MV/cm, at room temperature͒ has been reported in a
rf-sputtered Al2O3 gate ͑tox=39 nm͒.6 However, there is no
report on detailed analysis of current conduction mechanism
in an atomic-layer-deposited Al2O3 gate on SiC, if compared
with the same type of oxide deposited on Si ͑Ref. 10͒ or
nitrided SiO2 on SiC ͑Ref. 9͒ even though this type of oxide
has revealed better electronic properties than others. There-
fore, it is extremely important to shed some light on this area
by investigating and analyzing the possible current conduc-
tion mechanisms of Al2O3 gate on 4H SiC, so that a better
understanding of leakage current through this gate could be
obtained. In this letter, current conduction mechanisms, as a
͓JTFL=B͑VᐉG+1/tox ͔͒,
and
Child’s
law
͓JChild
=͑9␮␧ ␬V2 ͒/͑8t3 ͔͒ ͑Fig. 1͒; where q, n0, ␮, B, ␧0, and ᐉ are
0
G
ox
electronic charge, density of thermal generated free carriers,
electronic mobility in Al2O3, a constant, permittivity at
vacuum, and a constant related to trap distribution and
temperature.12 The extracted slopes from each region at dif-
ferent temperatures have been presented in the inset of Fig.
1. For current conduction obeying Ohm’s law, the density of
thermally excited electrons from trap centers at Al2O3 bulk
a͒
Electronic mail: cheong@eng.usm.my
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0003-6951/2007/90͑16͒/162113/3/$23.00 90, 162113-1 © 2007 American Institute of Physics
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162113-2
Cheong et al.
Appl. Phys. Lett. 90, 162113 ͑2007͒
FIG. 1. ͑Color online͒ Value of slope S extracted from J-VG plot governs by
space-charge-limited ͑SCL͒ conduction mechanism, which consists of
Ohm’s law, trap-filled limited, and Child’s law as a function of temperature.
A typical SCL plot, with three distinct linear regions, measured at 323 K is
shown in the inset of Fig. 1. SOhm, STFL, and SChild are the slope of plots
obeying Ohm’s law, trap-filled limited, and Child’s law, respectively.
FIG. 2. ͑Color online͒ Typical Pool-Frenkel ͑PF͒ emission plots as a func-
tion of temperature. Six linear portions of the plots are labeled as P, A, B, C,
D, and E as the applied electric field increases. Four different ledges of the
plots are indicated as I, II, III, and IV. Filled symbol in P linear portion
−1/2
͑293 K͒ is fitted with a slope of 11.39 ͑MV/cm͒
and it corresponds to
dynamic dielectric constant ␧ of 6.65. The distributions of dynamic dielec-
r
tric constant ␧ ͑᭿͒ and dielectric constant extracted from high frequency
r
C-V curve ␬ ͑᭺͒ as a function of temperature are presented in the inset of
͑n0͒ is much larger than the density of injected electrons
from SiC as VG is applied. As temperature increases, elec-
trons located at deeper trap centers are much easier to be
excited. Therefore, more injected electron is required to
achieve the same density of the thermal generated electron.
Hence, the slope of this limiting curve is deviated from the
ideal slope of 1, which is demonstrated in Ohm’s law, and
approaches TFL mechanism. The TFL mechanism also oc-
curs at higher VG that enables more injected electrons to be
supplied and then captured in trap centers located at Al2O3
bulk. In this trapping process, charge compensation is not
happening. The transition of Ohm’s law to TFL happens at a
crossover or onset voltage, Von. At this voltage, electron tran-
the figure.
duction band or it is termed as Poole-Frenkel ͑PF͒ emission,
where six clearly defined linear regions ͑labeled as P, A, B,
C, D, and E͒ could be identified in the PF plots ͑Fig. 2͒. This
bulk-limited conduction is mathematically expressed as J
ͱ
=͑qNc␮͒E exp͕͓−q͑␾t− qE/␲␧ ␧ ͔͒/͓KT͔͖; where Nc, ␧r,
r
0
␾t, and K are the density of states in the conduction band
1/2
r
͑EC͒, dynamic dielectric constant ͑refractive index, n=␧ ͒,
trap energy level, and Boltzmann’s constant, respectively.
From the slope of the linearity, ␧ has been extracted and the
r
deviation of the ␧ values, due to the six different slopes in
r
one temperature, is presented in the inset of Fig. 2. The self-
sit time ͑␶ ͒ is equal to dielectric relaxation time and its
e
consistent ␧ values at different temperatures are comparable
calculated room-temperature value is approximately equal to
r
with those reported by Huang and Hwu5 and Wolborski et
al.13 and this has ensured that PF emission is the dominant
conduction mechanism in the investigated electric field. The
7.23ϫ10−7 s. By knowing ␶ and Von, ␮ ͑␶e=t2 /␮Von͒ has
e
ox
been deduced ͑␮=4.17ϫ10−6 cm2/V/s͒. Subsequently, with
these parameters, the calculated n0 value ͑n0=SOhmtox/q␮͒ is
equal to 1.52ϫ1018 cm−3; where SOhm is the slope of J-VG
plot governed by Ohm’s law. As temperature increases, the
slope of TFL decreases; indicating that a lesser VG is applied
for the injected electron to fully fill the trap centers at the
Al2O3 bulk. When VG further increases, trap-free space
charge limited or Child’s law dominates the conduction
mechanism. This type of mechanism is only observed at
323 KഛTഛ393 K. The reason for this observation, limited
to this specific T, is still unknown and more detailed inves-
tigation is needed. The crossover voltage from TFL to
Child’s law is defined as VTFL, indicating that beyond this
voltage any injected electrons may not be trapped in the bulk
because all of the traps have already been filled. As tempera-
ture increases from 323 to 383 K, the extracted VTFL de-
creases from 1.725 to 1.422 V. This observation may be as-
sociated with the earlier explanation of the relationship
between the increase of temperature and the change of TFL
slope, where a lower VG is required to fully fill the traps at a
higher temperature. This may also lower the VTFL of the sub-
sequent trap-free conduction by Child’s law.
extracted ␧ values are in agreement with those ␬ values
r
obtained from high frequency C-V measurement ͑inset of
Fig. 2͒. Coincidently, the linear portions of A–E in the FP
plot are also revealed in FN plots ͑Fig. 3͒. This suggests that,
besides FP emission, leakage current at this electric field is
also attributed to the tunneling of electron through a triangu-
lar energy barrier. The extracted effective barrier height, ⌽B,
between EC of SiC and Al2O3 is associated with the slope of
linear portion in the FN plots,6,9,14 and it is affected by E and
T ͑inset of Fig. 3͒. In this study, the ⌽B value is lower than
that obtained from soft x-ray photoelectron spectroscopy
͑⌽B=1.5 eV͒.15 As E further increases, the combination of
these mechanisms may eventually break the dielectric at
E=EHDB
.
Besides the linearity revealed in PF and FN plots, four
ledges ͑labeled as I, II, II, and IV͒ located at different E have
also been recorded. The observation of the saturated current
in the ledges are probably caused by an equilibrium of
charges between injected electron from SiC into trap centers
residing at the triangular energy barrier of Al2O3 ͑associated
with FN tunneling͒ and the emission of the trapped electron
Beyond the SCL region, leakage current is due to field-
enhanced thermal excitation of trapped electron into the con-
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at the same energy level to EC of Al2O3 ͑associated with PF
129.21.35.191 On: Sun, 21 Dec 2014 19:44:22

162113-3
Cheong et al.
Appl. Phys. Lett. 90, 162113 ͑2007͒
FIG. 3. ͑Color online͒ Typical Fowler-Nordheim ͑FN͒ tunneling plots as a
function of temperature. Five linear portions of the plots are labeled as A, B,
C, D, and E as the applied electric field increases. Four different ledges of
the plots are indicated as I, II, III, and IV. Filled circle in the A linear portion
͑293 K͒ is fitted with a slope of 1.89ϫ107 V/cm and it corresponds to
effective barrier height ⌽B of 0.99 eV. The distribution of ⌽B as a function
of temperature is presented in the inset of the figure.
FIG. 4. Relationship among temperature, electric field, and current conduc-
tion mechanism through atomic-layer-deposited Al2O3 gate on n-type 4H
SiC. The abbreviations FN, PF, TFL, SE, Child’s, and Ohm’s represent
Fowler-Nordheim tunneling, Poole-Frenkel emission, trap-filled limited,
Schottky emission, Child’s law, and Ohm’s law conduction processes. Ar-
row indicates the possible region where current conduction is dominated by
SE.
emission͒, assuming that the trap centers are donorlike.
When higher E is applied, either PF or FN will dominate the
conduction mechanism until the injection and emission of
electrons is in equilibrium, so that another ledge will be re-
corded. These combination leakage paths can be considered
as trap-assisted tunneling with the trap centers located dis-
cretely at four different energy levels in the dielectric.9
Besides the above-mentioned conduction mechanisms,
SE has also been evaluated in this sample. From the linear
portion of the SE plot ͑ln͑J/T2͒ vs E1/2͒, ⌽B and ␧r, as a
function of T, are obtained ͑not shown͒.5 The calculated ⌽B
value ranges from 1.08 eV at 293 K down to 0.83 eV at
413 K. This result is comparable with those shown in the
tion mechanisms depended on applied electric field and tem-
perature.
One of the authors ͑K.Y.C.͒ would like to acknowledge
the support of The Ministry of Higher Education, Malaysia,
through the Fundamental Research Grant Scheme ͑FRGS͒.
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has been systematically performed and reported. The con-
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